Compact raman generator with synchronized pulses

ABSTRACT

According to an embodiment of the disclosure, a Raman generator includes a Raman medium and one or more optical elements. The Raman medium is configured to receive a pump pulse at a first wavelength and shift at least a portion of the pump pulse energy or power into a Stokes-shifted pulse at a second wavelength. The one or more optical elements are configured to synchronize one or more subsequent passages of the Stokes-shifted pulse through the Raman medium with one or more subsequent pump pulses at the first wavelength. The synchronized passage of the Stokes-shifted pulse and one or more subsequent pump pulses through the Raman medium increases a power of the Stoke-shifted pulse.

TECHNICAL FIELD

The present disclosure is directed in general to lasers and morespecifically to a nonlinear wavelength conversion using stimulated Ramanscattering.

BACKGROUND OF THE DISCLOSURE

A variety of laser configurations are known. However, some of theseinclude components that unnecessarily increase the size and complexityof particular laser configurations. Further, some laser configurationshave unacceptable alignment requirements.

SUMMARY OF THE DISCLOSURE

To address one or more of the above deficiencies of the prior art, aRaman generator is provided with a Raman medium and one or more opticalelements. The Raman medium is configured to receive a pump pulse at afirst wavelength and shift at least a portion of the pump pulse energyor power into a Stokes-shifted pulse at a second wavelength. The one ormore optical elements are configured to synchronize one or moresubsequent passages of the Stokes-shifted pulse through the Raman mediumwith one or more subsequent pump pulses at the first wavelength. Thesynchronized passage of the Stokes-shifted pulse and one or moresubsequent pump pulses through the Raman medium increases a power of theStoke-shifted pulse.

Certain embodiments may provide various technical advantages dependingon the implementation. For example, a technical advantage of someembodiments may include the capability to make multiple passes of thepump and Stokes-shifted beams through the Raman medium using an opticalarrangement that will allow a long total path length through the crystalin a compact package characterized by a significantly shorter length. Atechnical advantage of other embodiments may include the capability toprovide a compact Raman generator that is about 28 cm in length with atotal optical path length that is about 90 cm. Another technicaladvantage may include the capability to have nine passes through a Ramancrystal with only five components. Yet another technical advantage mayinclude the ability to synchronize Stokes-shifted pulses with successivepump pulses to efficiently convert a train of pump pulses into a trainof Stokes-shifted pulses.

Although specific advantages have been enumerated above, variousembodiments may include some, none, or all of the enumerated advantages.Additionally, other technical advantages may become readily apparent toone of ordinary skill in the art after review of the following figuresand description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsadvantages, reference is now made to the following description taken inconjunction with the accompanying drawings, in which like referencenumerals represent like parts:

FIGS. 1A through 1D illustrate aspects of three general architecturesfor Raman devices;

FIG. 2A through 2C illustrates teaching concepts for embodiments of thedisclosure;

FIG. 3 illustrates a compact Raman generator according to an embodimentof the present disclosure;

FIG. 4 illustrates a synchronously pumped ring resonator according to anembodiment of the present disclosure;

FIG. 5 illustrates a synchronously pumped linear resonator, according toan embodiment of the present disclosure.

DETAILED DESCRIPTION

It should be understood at the outset that, although example embodimentsare illustrated below, the present invention may be implemented usingany number of techniques, whether currently known or not. The presentinvention should in no way be limited to the example implementations,drawings, and techniques illustrated below. Additionally, the drawingsare not necessarily drawn to scale.

This disclosure pertains to nonlinear wavelength conversion by usingstimulated Raman scattering (SRS) in crystals or other classes of Ramanmedia. According to this wavelength-conversion process, a portion of aninitial “pump” laser beam generates a signal beam having a wavelengththat is longer than the pump wavelength, and where this wavelengthdifference is determined by a characteristic frequency shift, or “Stokesshift,” of the Raman medium. A principal objective of certainembodiments of the disclosure is to make multiple passes of the pump andStokes-shifted beams through the Raman medium using an opticalarrangement that will allow a long total path length through the crystalin a compact package characterized by a significantly shorter length.Three approaches for achieving multiple passes are provided. All threeapproaches are particularly advantageous if the pump beam compriseseither a single sub-nsec pulse or a continuous train of such pulses.

As described herein, according to one embodiment, a compact,alignment-stable, multi-pass Raman generator accommodates a singlesub-nsec pump pulse. An exemplary manifestation of this disclosureprovides a total optical length of approximately 1 m in a package havinga maximum dimension of approximately 30 cm. Additionally, in order tominimize the number of components to implement such a multi-pass scheme,certain embodiments are designed to have nine passes through a Ramancrystal with only five components. Another feature of the certainembodiments is a design that minimizes the alignment sensitivity andcomplexity, while also recognizing that the pump and Stokes-shiftedbeams must generally be properly oriented relative to the crystal axes.Such is accomplished in certain embodiments with only two lenses and twolens-Porro prism combination components that require alignment relativeto the Raman crystal.

As also described herein, according to another embodiment, asynchronously pumped Raman ring resonator is designed to accommodate acontinuous train of sub-nsec pulses. This embodiment provides a designfor resonator optics that allows the Stokes-shifted signal to makemultiple passes through the ring resonator where the time betweensuccessive passes is tuned to match the inter-pulse period of thepump-pulse train. For each pass through the resonator, theStokes-shifted pulse passes through the Raman medium synchronously withsuccessive pump pulses. This allows the Stokes-shifted pulse to increasein power until it efficiently converts the subsequent train of pumppulses into a train of Stokes-shifted pulses. If the pump pulse train ismodulated by a temporal envelope, such as a temporal burst of pumppulses 2 to 100 nsec long at a burst rate of 10 kHz, this embodimentwill yield individual sub-nsec Stokes-shifted pulses at repetitionfrequency of 10 kHz.

As also described herein, according to yet another embodiment, asynchronously pumped linear Raman resonator is designed to operate in amanner similar to the synchronously pumped Raman ring resonator. Morespecifically, the Stokes-shifted pulse passes through the Raman mediumsynchronously with successive pump pulses; however, rather than bypassthe Raman medium for subsequent passage with a synchronized pulse, theStokes shifted pulse passes directly through the Raman medium in alinear fashion for subsequent passage with a synchronized pulse.

Stimulated Raman scattering is a well known means for shifting a laserwavelength from that of a pump laser to a longer wavelength. If the pumplaser happens to be tunable, the SRS process can shift the tunabilityfrom the pump-laser wavelength range to a longer wavelength range.Certain embodiments of the present disclosure are particularly useful inmeeting requirements for multiple wavelengths in the mid-infraredwavelength range, roughly from 2 to 5 μm.

Particular applications in the mid-infrared wavelength range requirecontinuous tuning while other applications only require a few discretewavelengths located in atmospheric transmission windows. However,solid-state crystals for mid-infrared (mid-IR) applications (wavelength2 μm to 5 μm) have maximum lengths around 10 cm. Therefore, a problemarises as efficient Raman conversion often requires lengths of severalmeters.

Given such problems, certain embodiments teach how optical beams can befolded and routed through a single crystal for many passes in a compactpackage with minimal alignment sensitivity. According to one embodiment,a compact, alignment-stable, multi-pass Raman generator accomplishesthis performance by using relay imaging between successive passes, whichperiodically compensates for diffractive spreading of the pump andStokes-shifted beams, and by using Porro prisms to fold the beamsbetween successive passes. Porro prisms are particularly suitable forsuch folding functions because they are alignment insensitive in onedirection. According to another embodiment, a synchronously pumped Ramanring resonator comprises a compact ring resonator containing only fouralignment-sensitive reflecting surfaces, two of which are dichroics orpolarization beam splitters (for the case in which the pump andStokes-shifted beams are orthogonally polarized) and two of whichprovide total reflection at the Stokes-shifted wavelength. According toyet another embodiment, a synchronously pumped linear Raman resonatoroperates in a manner similar to the synchronously pumped Raman ringresonator. However, rather than bypass the Raman medium for subsequentpassage with a synchronized pulse, the Stokes shifted pulse passesdirectly through the Raman medium in a linear fashion for subsequentpassage with a synchronized pulse.

To provide additional context for embodiments of the disclosure, threegeneral architectures for Raman devices are described below: a singlepass Raman generator, a Raman resonator, and a multi-pass Ramangenerator.

The first general architecture for Raman devices, a single pass Ramangenerator 100, is shown in FIG. 1A. The Raman generator 100 includes apump laser 105, a Raman medium 110, and one or more relay imaging opticlenses 115 a, 115 b. The Raman generator 100 is configured to pass apump beam through a portion of the Raman medium 110 and each of thelenses 115 to generate a Stokes-shifted beam.

In the single-pass Raman generator 100, the pump laser 105 launches apump beam at wavelength λ_(p) into the Raman medium 110. TheStokes-shifted signal at wavelength λ_(s) builds up from spontaneousRaman scattering. For long enough path lengths, at least 50% of the pumpenergy can be converted to the Stokes-shifted wavelength. This approachis conceptually very simple, but the available Raman media for mid-IRwavelength generation require path lengths of several meters foracceptable Raman conversion efficiency, while crystal lengths arelimited to approximately 10 cm. Therefore, a single pass through asingle crystal is well below threshold and of essentially no utility inmost particular applications. One could employ a long string of Ramancrystals with relay optics between crystals to convey the optical beamsfrom one crystal to the next, but this would be hopelessly complex,large, and subject to multiple misalignment degrees of freedom.

The second general architecture for Raman devices, a Raman resonator150, is shown in FIG. 1B. The Raman resonator 150 encloses the Ramanmedium 110 between two mirrors 125 a, 125 b that reflect theStokes-shifted wavelength back and forth in the Raman medium 110. As inany laser resonator, the mirrors 125 a, 125 b are designed such that themultiple passes overlap in the Raman medium, and they reflect theStokes-shifted wavelength as required. Mirror 125 a (M1) typically has100% transmission at the pump wavelength and 100% reflectivity at theStokes-shifted wavelength, and mirror 125 b (M2) has a lowerreflectivity, perhaps 50%, at the Stokes-shifted wavelength. The Ramanconversion builds up from spontaneous Raman scattering, and if thedesign is properly implemented Raman conversion efficiencies >50% areroutinely achieved. One potential limitation of the Raman resonator 150for some applications is that the resonator necessarily reduces theallowable bandwidth of the Stokes-shifted output beam, depending on thefinesse (or Q-factor) of the optical resonator. In other applications,this spectral narrowing might be a feature.

A limitation of the Raman resonator 150 that is addressed by certainembodiments of the present disclosure arises because desired conversionefficiencies of approximately 50% or more are only possible if the pumppulse length is much greater than the round-trip propagation timethrough the resonator. This transient phenomenon can be understood inthe following way. Raman-laser operation begins with very weakspontaneous emission that is amplified as it propagates back and forthbetween the resonator mirrors. With a sufficiently long pump-pulselength, the Stokes-shifted signal is allowed to make a large number ofround trips through the Raman medium, to the point that the energyconversion efficiency from the pump to the Stokes-shifted wavelength canreach practical levels of approximately 50% or more. However, if thepump-pulse duration is not long enough to allow that many round-trips,the conversion efficiency suffers. As a specific example, if theresonator optical length is 15 cm, the round-trip time will beapproximately 1 nsec, and the pump pulse length will have to be at leastapproximately 20-30 nsec for effective Raman conversion. This clearlyrules out applications for a single sub-nsec pump pulse. Embodiments ofthe compact multi-pass Raman generator disclosed herein address thislimitation of the Raman resonator in which the pump and Stokes-shiftedsignal propagate together throughout all of the multiple passes. As longas the product of the pump intensity and crystal length is sufficientlyhigh, as described in certain embodiments, very short pulse lengths canbe accommodated. The case of a pump waveform comprising a long sequenceof sub-nsec pulses is addressed by the synchronously pumped Raman ringresonator (described with reference to FIG. 4), or the synchronouslypumped Raman linear resonator (described with reference to FIG. 5),which essentially stretch the pump into a long train of pulses with thetotal temporal length of the pulse train being long enough to avoid thetransient issue mentioned above.

The third general architecture for Raman devices, a multi-pass Ramangenerator (MPG) 170, is shown in FIG. 1C. The multi-pass Raman generator170 shown in FIG. 1C is a modification of the arrangement of FIG. 1Asuch that the pump and Stokes-shifted beams are folded around so theymake multiple passes through the same crystal. This type of Ramangenerator has been employed for more than thirty years, and a gooddescription has been published by Trutna and Byer in “Multi-pass Ramangain cell,” Appl. OPT 19, 301 (1978). The MPG provides a long opticalpath length with periodic refocusing to enhance Raman conversion. TheMPG looks similar to the Raman resonator 150 of FIG. 1B. However, asseen in FIG. 1C, the mirrors 123 a, 123 b are curved and they have highreflectivity for all wavelengths of interest. The mirror spacing mayalso be adjusted with respect to the radii of curvature such that theresonator meets the well known conditions for stability. Rather thanhaving all passes overlap in the Raman medium as is the case for FIG.1C, in certain embodiments of the disclosure, as will be detailed below,reflectors may be designed to provide multiple passes that do notoverlap.

MPG operation is based on the fact that a light beam injected off-axisinto such a stable optical resonator bounces back and forth between themirrors, and as the beam makes successive passes through the resonator,the location where the beam reflects off the mirrors systematicallymoves around the mirror surface, typically forming a circular path nearthe rim of the mirrors with the center of the circle aligned along thelongitudinal symmetry axis of the two-mirror resonator. That is, thebeam spot “walks” around the mirror surface before being coupled out.FIG. 1D illustrates representative patterns that can be produced. Afterthe desired number of round trips, the beam is deflected out of the MPGby means of a pick-off mirror or some similar approach. Trutna and Byermade a gas cell Raman medium that had 25 passes.

MPGs have only been implemented with gaseous Raman media, which canoccupy as large as volume as can be designed to contain the requiredhigh-pressure gas. Trutna and Byer constructed a cell that was nearly 4m long, the mirror radii of curvature were 2 m, and the mirror diameterswere 12.7 cm.

Solid-state Raman crystals cannot be made arbitrarily large, and theyare typically limited to cm-scale transverse dimensions and lengths ofapproximately 10 cm. For high-power operation (areas of interest forparticular embodiments), solid-state laser media typically have ahigh-aspect ratio rectangular cross section for effective heat removalwith minimal thermo-optical distortions. For example, a crystallineRaman medium might have dimensions of 1×10×100 mm. To adapt the MPGconcept to such a crystalline slab, the 2-D pattern of mirrorreflections would have to be collapsed into a single plane, in whichcase all passes through the Raman medium would overlap at the same axiallocation at the center of the Raman medium. This would greatly enhancethe beam intensities at that common focus, such that the SRS interactionlength would be approximately equal to the confocal parameter for thegeometry used. Given the short length of the Raman medium and the shortmirror focal lengths, the effective interaction length might extend justa few centimeters on either side of the focus. In addition, the veryhigh intensity at this focus would run a serious risk of bulk damage. Incontrast to such scenarios, certain embodiments of the disclosure teachhow to maintain essentially constant beam intensity throughout theentire interaction length, such that the intensity can be much lowerwhile still achieving a sufficiently high intensity-length product togenerate efficient Raman conversion. Moreover, certain embodiments teachhow the optical power and the heat load can be distributed such thatthey essentially fill the entire Raman-crystal volume. This allows moreeffective heat removal and, hence, power scaling.

Embodiments of the present disclosure simultaneously fold a pump beamand Raman-shifted Stokes beam so that the two beams remain collinear andthey make multiple passes through the Raman generator. Although incertain embodiments disclosed herein the Raman medium will be describedas a crystal, other media may avail from teachings of the disclosure. Incertain embodiments, the total path length can be approximately 1 m ormore. Additionally, if longer path lengths are required for specificapplications, wider crystals may be used or several of the describedRaman modules may be arranged in series to provide the total path lengthrequired.

Configurations for certain embodiments may be designed to take intoconsideration certain requirements:

-   -   1. Due to diffraction, the pump and Stokes-shifted beams will        spread as they propagate along the multi-pass architecture. If        this spreading is not compensated, the average beam intensities        will monotonically decrease along the propagation length,        reducing the Raman conversion efficiency. Hence, embodiments of        the present disclosure employ an optical scheme to limit the        diffractive beam spreading to a tolerable level.    -   2. For crystalline Raman media, it is often necessary that the        optical beam propagation directions and the beam polarizations        must be properly oriented relative to the crystalline axes.        Therefore, embodiments of the present disclosure are configured        such that the multiple-passes through the Raman medium are        parallel (or anti-parallel) to each other, such that if one beam        is properly oriented, all beams are.    -   3. The overall multi-pass architecture is capable of being        folded to form a compact package, with a goal of a maximum        dimension being ˜30 cm.

In order to comprehend the multi-pass architecture of certainembodiments of the disclosure, it is best to begin by considering theconceptually simple case of two Raman crystals 205, 210 in series, as isschematically indicated in FIG. 2A. Lenses 215 a and 215 b form an imageof plane O at plane O′, and lenses 215 c and 215 d form an image of O′at O″. The objective of the relay imaging lenses 215 a-215 d is tocompensate for diffraction spreading. Specifically, any spreading thatarises as the pump and Stokes-shifted beams propagate is eliminated asan image of plane O is formed at plane O′. The same compensation arisesagain as the beams propagate from O′ to O″. Hence, the first of theabove-referenced requirements is satisfied, providing the propagationdistance from one image plane to the next corresponds to a high Fresnelnumber (i.e., minimal diffractive spreading). For a 100 mm crystallength and a refractive index of 2, the Fresnel number is about five fora wavelength of 2 μm. Although only two beam paths are shown in FIG. 2A(originating at the top and bottom of the crystal), it will beunderstood that any ray leaving the exit face of the first crystal 205can be imaged into a similar location in the entrance face of the secondcrystal 210.

This same type of relay imaging could be continued for additionalcrystals in series to achieve a sufficient mid-infrared (mid-IR)wavelength generation. However, such a scheme will often not be compact,as is specified in third requirement above. Additionally, in order forthe intensity of each beam to be high enough to yield efficient Ramanconversion, the beam spot sizes often need to be small, such as about0.5 mm to 1 mm diameter. Therefore, a single beam passing a single timethrough a Raman crystal typically uses only a small fraction of thetotal crystal volume. For example, if the beam size is 1 mm and thecrystal height is 10 mm, a single pass through the crystal uses only 10%of the total crystal volume. Hence, the beams often need to be routedback to the beginning of the first crystal 205 for multiple additionalpasses through the crystal for a total of about ten passes. Furthermore,when the beam returns, the beam can enter a different portion of thecrystal than any of the previous passes so that each pass is independentof all other passes.

A first step in approaching the desired multi-pass architecture is toconsider what happens if we place a mirror at the plane O′. This case isillustrated in FIG. 2B. This scheme allows the beams to go back throughthe crystal for a second pass; however, the first and second passesoverlap. Specifically, due to the inverting properties of the relaylenses, a ray at A propagating to the right is imaged to A′ at the imageplane O′; if that ray is reflected by 180 degrees, it returns to A.Further details below describe how the overlap problem can beeliminated, but for now we see that the scheme of FIG. 2B meets thefirst requirement above, namely that the input plane O of the Ramancrystal is imaged onto itself in one round-trip through the opticalpath.

The next step is to replace the mirror at O′ with a Porro prism 240, asshown in FIG. 2C. The Porro prism 240 is located such that the imageplane O′ has been moved to the center of the Porro prism 240, and thetwo total internal reflections (TIRs) from the prism 240 invert theimage plan by 180 degrees. In this configuration, the inverting propertyof the prism 240 prevents a beam at position A from returning exactlyupon itself. That is, this scheme takes a ray at position A and directsit as a ray at position A′. As shown in FIG. 2C, positions A and A′ arelocated symmetrically relative to the longitudinal symmetry axis of theoptical arrangement. Due to the imaging incorporated in this scheme,when ray at plane O propagates through the crystal to position A, thenpropagates to plane O′ and back to A′, and then propagates back to planeO, it will be an image of the original input beam, but the image will beinverted and displaced downward, thereby satisfying the firstrequirement above.

The next step is to add to the left side of the crystal anothercombination of two lenses and a Porro prism similar to the combinationshown in FIG. 2C, but with the vertical heights being reduced by thethickness of a single pass through the crystal. For example, if the beamheight, d, is 1 mm and the total crystal height is 10 mm, the second setof lenses and Porro prism will be 10% shorter than the first set. Thisleads to the arrangement shown in FIG. 3, where the two imaging lensesassociated with each Porro prism (e.g., the lenses shown in FIG. 2C) arenot shown for simplicity.

FIG. 3 illustrates a compact Raman generator 300 according to anembodiment of the present disclosure. Although certain details will beprovided with reference to the components of the Raman generator 300 ofFIG. 3, it should be understood that other embodiments may include more,less, or different components. Additionally, it should be understoodthat certain components have not been shown for ease of explanation.

The Raman generator 300 is generally shown receiving an input/pump pulse305 (e.g., received from any suitable pump laser) which is passedmultiple times through a laser crystal 310 and respectively reflectedback and forth through the crystal 310 by a first prism 340 and a secondprism 345. As an alternative, either or both of the prism 340 and theprism 345 may be replaced by an assembly of two flat reflectors orientedat 90 degrees to each other that function together in a manner similarto the two total internal reflection surfaces of the prism 340 and theprism 345. Depending on the details of the geometry, relay-imagingoptics may or may not be required. If the beam cross-section dimension dis large enough to ensure that the Fresnel range of the beam,L_(Fr 0)˜πd²/λ, exceeds the multi-pass interaction length L_(tot) insidethe crystal, i.e. L_(Fr)>L_(tot), then diffraction effects are minor fora collimated input pump beam and relay imaging is not necessary. If thiscondition is not satisfied, then relay imaging will be required.Although not shown in FIG. 3, relay imaging (for example, as shown inFIG. 2C) may be positioned between the laser crystal 310 and the prisms340, 345 for example, at the location indicated by arrows 315, 317.Additionally, in particular embodiments, a lens and a prism may beintegrated together. For example, the Porro prism 340 and lens 215 b ofFIG. 2C may be a combination structure. In certain embodiments, theRaman generator 300 includes a spectral filter 330 inserted in the beampath. In certain embodiments, the filter 330 limits the frequency spreadof the Stokes-shifted beam relative to the frequency of the pump beam.Frequencies outside the filter band-pass are blocked each round-tripthrough the crystal 310, leaving only the desired frequency components.In certain embodiments, the filter 330 can be a Fabry-Perot etalonhaving a narrow passband, or a narrowband birefringence filter, or someother type of filter that provides the required frequency selection.Assuming the pump laser has a broad spectral bandwidth as compared tothe filter transmission bandwidth, the filter 330 will have negligibleimpact on the pump beam as it passes through the Raman generator 300.

In the multi-pass scheme of FIG. 3, there are a total of nine passesthrough the Raman crystal 310. In this embodiment, the final reflectionat the left side propagates directly along the symmetry axis of thecrystal 310 and the Porro prism 340. The Porro prism 340 is designed tohave an AR-coated flat surface at its right side, which allows thisninth-pass beam to propagate right through the prism 340 without TIR andto exit the prism 315, yielding the output signal 390 to the right.Although the sketch in FIG. 3 only shows the pump beam, theStokes-shifted beam would follow the same path.

In particular embodiments, the nominal beam diameter is 0.7 mm, and thespacing between successive passes is 1 mm. Nine beam passes cantherefore fit within an overall crystal height of 10 mm. Assuming thecrystal 310 has a length L and a refractive index n, and referring backto FIG. 2A, the lens focal lengths and spacing between crystals 205-210can be determined. Given that planes O, O′, and O″ are relay images ofeach other that are sequentially spaced by 4f (f is the lens' focallengths) and the spacing between crystals is defined as L_(x), thefollowing equation applies:

$\begin{matrix}{{\frac{2L}{n} + L_{x}} = {8f}} & (1)\end{matrix}$which can be solved for L_(x)/2:

$\begin{matrix}{\frac{L_{x}}{2} = {{4f} - \frac{L}{n}}} & (2)\end{matrix}$

With lens focal lengths of 35 mm, a crystal 205 length of 100 mm, and acrystal refractive index of 2, and referring back to FIG. 2C, Equation(2) yields the distance from the right end of the crystal 205 to theimage plane O′ of L_(x)/2=90 mm. With a nominal 10 mm between lens 215 band the image plane O′ and 70 mm between lens 215 a and lens 215 b, lens215 a is about 10 mm from the right end of the crystal 205. Accordingly,the vertical extent of all nine beams as they enter lenses is about 10mm, so this lens set is operating at or about f/3.5, which is arelatively simple lens to design. Assuming a comparable opticalarrangement on the left side of the crystal 205, the total physicallength of the Raman generator module 205 schematically shown in FIG. 3is about 28 cm, while the total optical path length is about 90 cm.

As alluded to above, in certain embodiments a lens and a Porro prism canbe combined to form a single integrated lens-prism. Therefore, eachprism can be configured to have a spherical entrance face with a radiusof curvature that makes it function as a 35 mm (or other) focal lengthlens. Utilizing an integrated lens-prism eliminates one component oneach side of the crystal. Thus, the entire Raman generator would theninclude only five components: the crystal, two lenses, and twolens-prism combinations. In embodiments where the filter 330 is includedfor narrow-band applications, the Raman generator includes a total ofsix components.

In certain embodiments, the Raman medium is Potassium GadoliniumTungstate (KGW) with a Raman gain of about 1 cm/GW. In otherembodiments, the Raman medium may be other materials. When the laserpump includes a pump intensity of 325 MW/cm² (i.e. 1 mJ, 0.5 nsec, 0.7mm beam diameter) and the Raman generator 300 as a total multi-pass pathlength of 90 cm, the gIL product is 45, which exceeds the Ramanthreshold. Therefore, the Raman generator module 300 will reachthreshold. Additionally, three Raman generators 300 in series will reachabout five times that threshold, which yields good conversionefficiency. Alternatively, the pump and signal beams can be folded in adirection normal to the plane of FIG. 3, and then the same optics oneither end of the Raman medium can be used to make an additional 9 morepasses through the crystal. Folding in this direction one more timeyields another 9 passes. Hence, by tripling the package size in theplane normal to FIG. 3, the final package will operate at about fivetimes threshold, while maintaining all of the benefits described above.The principal disadvantage of this final scheme is that the thermalgradients in the direction of the three layers will increase byapproximately a factor of 3 relative to the single-layer case. Thisdisadvantage can be addressed by using three separate crystal platesaligned in parallel and separated by cooling inserts.

In certain embodiments, the design for the Raman generator 300 can alsoinclude alignment of the beams with respect to the crystal axes, whichaddresses the second requirement above. The Porro prisms 340, 345 can befabricated with relatively tight tolerances on the 90 degree anglebetween the two TIR surfaces. As a result, all of the beam paths in themulti-pass scheme can be either parallel or anti-parallel to each other.Therefore, once the input beam is aligned to have the correctpropagation direction and polarization relative to the crystal axes, allof the beam paths may also be so aligned. Furthermore, conventionalPorro prisms have a retro-reflection property such that they can bemisaligned by rotating within the plane of the drawings without alteringany of the beam alignments. The only potential misalignment is in adirection normal to the plane of the figures. Having only a singlecritical alignment at each end of this configuration makes it highlyviable for hardware implementation.

FIG. 4 illustrates a synchronously pumped ring resonator 400 accordingto an embodiment of the present disclosure. Although certain detailswill be provided with reference to the components of the ring resonator400 of FIG. 4, it should be understood that other embodiments mayinclude more, less, or different components. The ring resonator 400 ofFIG. 4 includes a Raman medium 410, a plurality of reflective surfaces415 a-415 d, a spectral filter 420, and an etalon 425. As described inmore detail below, the ring resonator 400 in this embodiment isconfigured to receive input pump pulses and to generate outputStokes-shifted pulse(s). In particular embodiments, there may also be aresidual pump beam as indicated by dashed line 407.

This embodiment can be beneficial for the specific pump waveform of amode-locked pulse train. The mode-locked waveform is represented as aperiodic sequence of short pulses at a high pulse repetition frequency(PRF). For typical conditions, the pulse duration, τ_(p), is a few psec,with a time interval T between pulses of a few nanoseconds. Theinter-pulse period is selected to match the resonator transmission time,T=c/Lr, where Lr is the one-way distance around the ring resonator.

A laser pump provides a train of light pulses 405 at a pump wavelengthλ, to the ring resonator 400. For example, the laser pump can provide apulse every 10 ns with a pulse duration (τ_(P)) of 1 psec. The pulsepropagates through a first reflective surface (e.g., a dichroic mirror)415 a. The first mirror 415 a can be disposed at a 45° angle withrespect to the path of the pulse. Additionally, the first mirror 415 amay be configured to allow the pulse to pass through the mirror 415 a.The pulse is then passed through a transverse mode selector 430, whichlimits angular spread into the Raman medium 410. The Raman medium 410outputs a low intensity Stokes-shifted beam, such as with an efficiencyof 10⁻⁸, to the second mirror 415 b. The second mirror 415 b can beconfigured to pass a portion of the Stokes-shifted beam and reflect theremainder of the Stokes-shifted beam along with all other beams. Forexample, the second mirror 415 b can be a dichroic mirror. The beamtraverses a path that includes the third mirror 415 c, spectral filter420, etalon 425, and fourth mirror 415 d back to the first mirror 415 a.

The spectral filter 420 filters undesired wavelengths. For example, thespectral filter 420 is configured to prevent the generation ofhigher-order Stokes-shifted pulses that otherwise could be generated anddeplete the first-Stokes intensity when the Raman conversion efficiencybecomes high.

The etalon 425 enables a selection of modes for the Stokes-shifted beam.The etalon 425 can be a Fabry-Perot etalon configured to select aspecific Raman wavelength within the overall Raman gain band, such as ifnarrowband operation of the Raman converter is needed.

The path length is configured such that the Stokes-shifted beamsubstantially coincides with the next pulse generated by the pump laser.The Stokes-shifted beam continues to cycle through the ring resonator400, coinciding with the subsequent input pump pulses. Each time theStokes-shifted beam passes through the resonator a portion istransmitted through the second mirror 415 b to form a Stokes-shiftedoutput pulse at wavelength λ_(s).

The Raman process efficiency is controlled by the pump intensity insidenonlinear Raman medium 410. The Raman process efficiency can be boostedup exponentially if the pump power increases. Since the mode-lockedoptical power is concentrated as a sequence of short high-power pulses(that is, with a temporal duty cycle <<1), the peak power within thepulse can greatly exceed the average power by two to three orders ofmagnitude, which results in improved Raman conversion efficiency.

For a mode-locked pump, Raman conversion occurs in the forward directionin a specific regime when short pump and Stokes-shifted pulses runthrough the nonlinear medium together, with good overlap both in spaceand in time. The Raman conversion occurs as a result of the interactionbetween these two pulses only, having no direct participation from otherpulses. The lifetime of the medium vibrations responsible for the Ramanprocess, which is given by the inverse of the Raman bandwidth Δν_(R), isusually much shorter than the interval T between pulses, (TΔν_(R))⁻¹<<1.As such, no memory is required in the medium about events induced bypreceding pulses when new pulses arrive, other than that precedingpulses have systematically contributed to the energy contained in theStokes-shifted pulse.

In certain embodiments of the pulse-train Raman mode in the ringresonator 400, the interaction between the pump and Stokes-shifted beamscan be as efficient as the steady-state interaction between a continuouswave (CW) pump and Stokes-shifted beams. In order to maintain thepulse-train efficiency of the pulsed waveform at a level comparable tothat of the CW waveform, the following conditions can occursubstantially simultaneously in certain embodiments:

-   -   1. The pulse duration τ_(p) can be long, τ_(p)Δν_(R)>>1,        compared to the Raman vibrational lifetime.    -   2. The group velocity difference, ΔV_(gr)=V_(pump)−V_(stokes),        can be small enough after passing the Raman interaction length        L_(int), L_(int)(ΔV_(gr)/V_(gr))<<τ_(p)V_(gr), to neglect the        Raman-pulse temporal walk-off, which can happen for ultra-short        pulses because of medium dispersion.    -   3. The Group Velocity Dispersion (GVD) can be small enough to        avoid a peak-intensity reduction due to pulse spreading over the        interaction length, (d²k/dω²)<<τ_(p) ²/L_(int).

For most of the Raman-efficient solid-state media, these threeconditions are safely satisfied if the pulse duration τ_(p) exceedsabout 30 ps. For silicon (Si) crystals, for example, which are of basicinterest for mid-IR Raman conversion applications, the vibrationallifetime amounts to about thirty times less, 1/Δν_(R)˜1 ps. For 30 pstransform-limited pulses in Si, the walk-off distance where the pulsessynchronous at the input become time-separated by more than the pulseduration amounts to about 75 cm, which is much longer than any Sicrystal ever grown to date. Dispersion-induced pulse spreading in Si forwavelengths above 2 μm is pretty slow and measures in hundreds of metersfor 30 ps pulses, so it can be neglected as well. That is, a typicalmode-locking regime with about 30 ps pulses appears to be a “sweet spot”for Raman conversion, so that the pulse-train mode is automatically asefficient as the truly CW mode, but with conversion efficiency given bythe peak power instead of average power, which represents a two-to-threeorders of magnitude improvement in terms of effective Raman gain.

The conditions 1-3 listed immediately above optimize the generation ofthe Stokes-shifted pulse train. However, the scheme of FIG. 4 can stillbe applied if the conditions are not met. If the pulse duration isshorter than the lifetime, τ_(p)Δν_(R)<1, the synchronously pumped ringresonator can still function, but at higher power threshold because theRaman scattering will be developing in a transient regime. Moreover, thedispersion walk-off effect can be partially compensated by matching theround-trip time to the time interval between the pulses. This can bedone by tuning the ring-resonator length to account for the differencein propagation time for pump and Stokes-shifted pulses through thecrystal.

The ring resonator 400 yields a high Raman conversion efficiency evenfor conditions that do not produce a high conversion efficiency in asingle-pass Raman generator. Specifically, a single-pass Raman generatorilluminated by a pump intensity, I_(p), and containing a Raman mediumwith a Raman gain coefficient g and having a length of L_(int) will onlyachieve efficient conversion to the Stokes-shifted wavelength when thegain factor, M=gI_(p)L_(int), is sufficiently high, such as M>100. Theneed for this high gain factor arises from the fact that theStokes-shifted signal comprises the very low power spontaneous Ramannoise that amounts in typical conditions to about exp(−25) of the pumppower. This very weak noise is amplified by an exponential gain factorof M_(thr) (such as about 25) to reach the Raman threshold, and at leastfour to five times stronger than that to provide good conversionefficiency. For the best expected values of relevant parameters g=10cm/GW and L_(int)=10 cm in order to meet this condition, the pump peakintensity I_(p) should be about 1 GW/cm². The long interaction lengthL_(int)=10 cm establishes a limit on the focused-pump spot area A insidea bulk nonlinear medium, A>λL_(int)˜10⁻³ cm², which means that the peakpower for a single pulse should exceed 1 MW. This is almost four ordersof magnitude above the available peak power for standardpicosecond-range mode-locked lasers. Under these conditions, theorder-of-magnitude improvement given by the multi-pass geometry of thecompact Raman generator 300 of FIG. 3, which is very efficient fortypical nanosecond-range Q-switched laser pulses, still can beinadequate. The situation is quite different for the ring resonator 400.In this case, the Stokes seed power coincident with each pump pulse thatenters the Raman medium is greater than that of the Stokes seed for thepreceding pump pulse, and this progression continues until the Stokesseed finally reaches the level that it begins to extract at least 50% ofthe pump power on a single pass through the Raman medium.

In certain embodiments, the ring resonator 400 includes a ring resonatorlength that is matched to the resonator length of the pump laser. TheStokes-shifted pulse is excited by a pump pulse and travels togetherwith the pump pulse through the active medium 410. Then, theStokes-shifted beam returns back to the input after the resonatortransmission time. For matched cavity lengths, the Stokes-shifted pulsearrives at substantially the same moment as when the next pump pulse isdelivered. The returned Stokes-shifted pulse serves as a Raman seed forthe next pump pulse such that the stimulated scattering process for thesecond pump pulse starts from a strong Stokes-shifted signal instead ofa weak spontaneous seed. The Raman threshold drops down significantlyfor the second and subsequent pulses. For the ring resonator 400, thethreshold is defined by the condition of overall gain for Stokes-shiftedradiation per trip around the resonator: M_(thr)=l_(n)(1/R_(r)), whereR_(r) is the effective one-way transmission of the resonator controlledmostly by the out-coupler reflectivity R_(out), Fresnel reflections onsurfaces, and optical absorption in the optical components of thecavity. Practical values for the resonator transmission, R_(r) isbetween about 0.7 to about 0.9, provide good enough Raman conversionefficiency but at a Raman threshold that is reduced by two orders ofmagnitude.

The out-coupling can be achieved with a dichroic mirror 415 b thattransmits the pump and only a fraction of the Stokes-shifted beam.Alternatively, the out-coupling can be achieved using differentpolarization states for the pump light and the Stokes-shifted light inthe ring resonator 400. The two tilted optical surfaces that are crossedby the linearly polarized pump pulse would be set to transmit the pumppolarization but reflect the Stokes-shifted pulses, which would beforced to operate at the orthogonal polarization. Most of theRaman-active solid-state media allow pump and Stokes-shifted waves tointeract and form stimulated Raman scattering gain even if the two wavesare linearly polarized orthogonally to each other.

In certain embodiments, the ring resonator 400 includes a spatialfilter. The spatial filter can be set inside to reduce the Fresnelnumber of the resonator to a low value that helps limit theStokes-shifted output beam to a diffraction-limited output.

In certain embodiments, the active medium 410 can be a long waveguide.Such a waveguide is useful since a long waveguide can be made with avery small cross-sectional area but support high intensity of the pumpbeam over much longer distance than if the same pump beam were focusedinto a bulk Raman medium. Such waveguide technology has been matured inrecent years for crystalline Si; it is referred to as “ridge waveguide”technology. These waveguides are usually highly multimode because of thestrong index difference of the Si with respect to any cladding. For thisreason, the waveguides can support propagation of a few independentlaser paths, which is useful for scaling up output power of theconverter. If desired, an aperture in the ring resonator 400 can cut offany undesired higher-order modes for the generated Stokes-shiftedsignal, thereby providing single-mode operation for multi-mode andincoherent pumping.

FIG. 5 illustrates a synchronously pumped linear resonator 500,according to an embodiment of the present disclosure. Although certaindetails will be provided with reference to the components of the linearresonator 500 of FIG. 5, it should be understood that other embodimentsmay include more, less, or different components. The linear resonator500 of FIG. 5 includes a Raman medium 510, a plurality of reflectivesurfaces 515 a-515 b, a spectral filter 520, and an narrowband spectralfilter 525. As described in more detail below, the linear resonator 500in this embodiment is configured to receive input pump pulses 505 and togenerate output Stokes-shifted pulse(s) 590.

This embodiment offers many of the benefits of the ring resonator 400 ofFIG. 4, but with fewer parts and a simpler geometry. Specifically, aswith ring resonator 400, linear resonator 500 is configured to receivethe specific pump waveform of a mode-locked pulse train, as definedearlier in connection with ring resonator 400. Furthermore, linearresonator 500 is specified such that the inter-pulse period, T, of thepulse train matches twice the linear resonator transmission time,T=2c/Lr, where Lr is the one-way length of the linear resonator. In thiscase, a Stokes-shifted pulse that starts to grow under the influence ofa single pump pulse within the linear resonator will continue to growunder the influence of subsequent pump pulses until the Stokes-shiftedpulse grows to a level of at least 50% of the energy of a pump pulse.

In certain embodiments, the Raman generator 500 includes a spectralfilter 520 which filters undesired wavelengths. For example, configuringspectral filter 520 to efficiently pass the pump and Stokes-shiftedwavelengths but to block any longer wavelengths will prevent thegeneration of higher-order Stokes-shifted pulses that otherwise could begenerated and deplete the first-Stokes intensity when the Ramanconversion efficiency becomes high. The filter 520 can be abirefringence filter, or some other type of filter that provides therequired frequency selection.

In certain embodiments, the Raman generator 500 includes a narrowbandspectral filter 525 inserted in the beam path. The filter 525 enables aselection of modes for the Stokes-shifted beam. The filter 525 can be aFabry-Perot etalon, or some other type of filter that provides therequired frequency selection. Assuming the pump laser has a broadspectral bandwidth as compared to the filter bandwidth, the filter 525will have negligible impact on the pump beam as it passes through theRaman generator 500.

Modifications, additions, or omissions may be made to the systems,apparatuses, and methods described herein without departing from thescope of the invention. The components of the systems and apparatusesmay be integrated or separated. Moreover, the operations of the systemsand apparatuses may be performed by more, fewer, or other components.The methods may include more, fewer, or other steps. Additionally, stepsmay be performed in any suitable order. As used in this document, “each”refers to each member of a set or each member of a subset of a set.

To aid the Patent Office, and any readers of any patent issued on thisapplication in interpreting the claims appended hereto, applicants wishto note that they do not intend any of the appended claims or claimelements to invoke paragraph 6 of 35 U.S.C. Section 112 as it exists onthe date of filing hereof unless the words “means for” or “step for” areexplicitly used in the particular claim.

What is claimed is:
 1. A Raman generator comprising: a Raman mediumconfigured to receive a pump pulse at a first wavelength and shift atleast a portion of the pump pulse's energy or power into aStokes-shifted pulse at a second wavelength; and one or more opticalelements configured to synchronize one or more subsequent passages ofthe Stokes-shifted pulse through the Raman medium with one or moresubsequent pump pulses at the first wavelength to thereby increase apower of the Stokes-shifted pulse; wherein the Raman medium has a lengththat avoids achieving a Raman threshold for stimulated Raman scatteringduring a single pass through the Raman medium.
 2. The Raman generator ofclaim 1, wherein the one or more optical elements are configured tosynchronize the one or more subsequent passages of the Stokes-shiftedpulse with the one or more subsequent pump pulses by routing theStokes-shifted pulse from an output of the Raman medium back to an inputof the Raman medium.
 3. The Raman generator of claim 2, wherein the oneor more optical elements are configured to route the Stokes-shiftedpulse from the output of the Raman medium back to the input of the Ramanmedium by sending the Stokes-shifted pulse directly through the Ramanmedium.
 4. The Raman generator of claim 2, wherein the one or moreoptical elements are configured to route the Stokes-shifted pulse fromthe output of the Raman medium back to the input of the Raman medium bybypassing the Raman medium.
 5. The Raman generator of claim 1, whereinthe one or more optical elements are configured to pass theStokes-shifted pulse multiple times along a same direction through theRaman medium.
 6. The Raman generator of claim 1, wherein the one or moreoptical elements are configured to synchronize the Stokes-shifted pulsewith multiple subsequent pump pulses during multiple subsequent passagesof the Stokes-shifted pulse through the Raman medium.
 7. The Ramangenerator of claim 1, wherein the one or more optical elements compriseat least two mirrors.
 8. The Raman generator of claim 1, wherein the oneor more optical elements comprise at least four mirrors.
 9. The Ramangenerator of claim 1, further comprising; a transverse mode selectorconfigured to limit an angular spread of the Stokes-shifted pulse intothe Raman medium.
 10. The Raman generator of claim 1, furthercomprising, a spectral filter configured to control a spectral width ofthe Stokes-shifted pulse.
 11. The Raman generator of claim 1, furthercomprising, a spectral filter configured to suppress multi-Stokes signalgeneration beyond a specified number of Stokes shifts.
 12. The Ramangenerator of claim 1, further comprising: an optical element configuredto limit a diffractive beam spreading of a Stokes-shifted output beam.13. The Raman generator of claim 1, wherein the Raman generator isconfigured such that the Stokes-shifted pulse, after multiple passesthrough the Raman medium, has an energy that is greater than or equal tofifty percent of the energy of the pump pulse.
 14. A method comprising:passing a pump pulse at a first wavelength through a Raman medium toshift at least a portion of the pump pulse's energy or power into aStokes-shifted pulse at a second wavelength; and synchronizing one ormore subsequent passages of the Stokes-shifted pulse through the Ramanmedium with one or more subsequent pump pulses at the first wavelengthto thereby increase a power of the Stokes-shifted pulse; wherein theRaman medium has a length that avoids achieving a Raman threshold forstimulated Raman scattering during a single pass through the Ramanmedium.
 15. The method of claim 14, wherein synchronizing the one ormore subsequent passages of the Stokes-shifted pulse with the one ormore subsequent pump pulses comprises: routing the Stokes-shifted pulsefrom an output of the Raman medium back to an input of the Raman medium.16. The method of claim 15, wherein routing the Stokes-shifted pulsefrom the output of the Raman medium back to the input of the Ramanmedium comprises: sending the Stokes-shifted pulse directly through theRaman medium.
 17. The method of claim 15, wherein routing theStokes-shifted pulse from the output of the Raman medium back to theinput of the Raman medium comprises: bypassing the Raman medium with aplurality of optical elements.
 18. The method of claim 14, wherein theStokes-shifted pulse is synchronized with multiple subsequent pumppulses during multiple subsequent passages of the Stokes-shifted pulsethrough the Raman medium.
 19. The method of claim 14, furthercomprising: limiting diffractive spreading of at least one of: the pumppulse and the Stokes-shifted pulse.
 20. The method of claim 14, whereinthe Stokes-shifted pulse, after multiple passes through the Ramanmedium, has an energy that is greater than or equal to fifty percent ofthe energy of the pump pulse.
 21. A system comprising: a laserconfigured to generate a pump pulse at a first wavelength and one ormore subsequent pump pulses at the first wavelength; and a Ramangenerator comprising: a Raman medium configured to shift at least aportion of the pump pulse's energy or power into a Stokes-shifted pulseat a second wavelength; and one or more optical elements configured tosynchronize one or more subsequent passages of the Stokes-shifted pulsethrough the Raman medium with the one or more subsequent pump pulses tothereby increase a power of the Stokes-shifted pulse; wherein the Ramanmedium has a length that avoids achieving a Raman threshold forstimulated Raman scattering during a single pass through the Ramanmedium.